1. Technical Field
The present invention relates to optical semiconductor elements that emit laser light and methods for manufacturing the same.
2. Related Art
A surface-emitting type semiconductor laser is one type of optical semiconductor elements that emit laser light. The surface-emitting type semiconductor laser is provided with a resonator formed in a direction orthogonal to a surface of the substrate, and emits laser light from the substrate. Compared to conventional edge-emitting type semiconductor lasers that use parallel cleavage surfaces of a substrate as a resonator, the surface-emitting type semiconductor laser has various favorable characteristics. For example, surface-emitting type semiconductor lasers have low power consumption, and are capable of high-speed direct modulation, and a two-dimensional laser array structure can be readily formed with surface-emitting type semiconductor lasers. Therefore, surface-emitting type semiconductor lasers are highly expected as light sources that are suitable for optical interconnections that handle a great amount of data. Currently, surface-emitting type semiconductor lasers with oscillation wavelengths being centered at the 850 nm range are coming into full scale application to modules (optical interconnection modules), and their further miniaturization and cost reduction are desired.
An optical transmission module, represented by a transmitter optical sub-assembly (TOSA), is equipped with a surface-emitting type semiconductor laser, a reflection member that reflects a portion of laser light emitted from the surface-emitting type semiconductor laser, such as, a diagonal glass, and a photodetector element that detects the laser light reflected by the reflection member. Driving current for the surface-emitting type semiconductor laser is feed-back controlled based on the detection result provided by the photodetector element. This structure realizes constant optical output driving (or automatic power control (APC)) that maintains constant the power of laser light emitted from the surface-emitting type semiconductor laser.
However, the optical transmission module described above requires highly accurate positioning in order to increase the coupling efficiency with respect to the photodetector element that detects laser light reflected by the reflection member. Also, because the optical transmission module requires the reflection member, the photodetecting element and other elements besides the surface-emitting type semiconductor laser, the number of components increases, and therefore the cost increases. In order to suppress the cost increase by reducing the number of components of an optical transmission module, an optical semiconductor element that integrates a surface-emitting type semiconductor laser and a photodetector element that monitors the amount of laser light emitted from the surface-emitting type semiconductor laser in one piece has been recently proposed.
For example, Japanese laid-open patent applications JP-A-2005-33106 and JP-A-2000-269585 describe an optical semiconductor element equipped with a photodetecting element provided above a surface-emitting type semiconductor laser. The structure of the optical semiconductor element described in JP-A-2005-33106 is briefly described below. The surface-emitting type semiconductor laser of the optical semiconductor element is formed by sequentially laminating, for example, an n-type first mirror composed of a semiconductor multilayer film, an active layer and a p-type second mirror composed of a semiconductor multilayer film. Also, the photodetecting element is formed by sequentially laminating, for example, an n-type first contact layer, a absorption layer, and a p-type second contact layer. Further, the optical semiconductor element is equipped with an electrode connected to the first mirror and an electrode connected to the second mirror of the surface-emitting type semiconductor laser, and an electrode connected to the first contact layer and an electrode connected to the second contact layer of the photodetecting element, wherein the electrode connected to the second mirror of the surface-emitting type semiconductor laser and the electrode connected to the first contact layer of the photodetecting element are electrically connected to each other.
When manufacturing optical semiconductor elements described in the aforementioned patent documents, it is noted that, unless a sufficient margin is secured in each of the manufacturing steps, the defect rate would become high and the manufacturing yield would lower, or optical semiconductor elements having high resistance that do not achieve the designed performance would be manufactured. For example, in the optical semiconductor element described in JP-A-2005-33106, the electrode connected to the second mirror of the surface-emitting type semiconductor laser and the electrode connected to the first contact layer of the photodetecting element are electrically connected to each other. These electrodes are formed with different materials in different steps. For this reason, when forming each of the electrodes, line disconnections may occur due to step differences formed unless a sufficient margin is secured in each mask for forming each of the electrodes.
Also, if metal composing each of the electrodes provided on the optical semiconductor element is oxidized, the bonding strength of each of the metal wires (bonding wires) to be connected to the electrodes may deteriorate, such that the metal wires would likely be detached, and the reliability of the element may lower. Furthermore, if the contact area of the electrode on the photodetecting element with respect to the first contact layer or the contact area with respect to the second contact layer is small, a problem occurs in that the resistance becomes higher and favorable characteristics (diode characteristics) would not be obtained.